Gravity and Weight — AQA GCSE Physics
Weight is the force of gravity on an object. It depends on mass and the gravitational field strength.
Mass and weight
- Mass is the amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
- Weight is the force acting on an object due to gravity, measured in newtons (N). Weight depends on the gravitational field strength where the object is.
The weight equation
$$W = m \times g$$
- W = weight (N)
- m = mass (kg)
- g = gravitational field strength (N/kg) — about 9.8 N/kg on Earth.
Weight changes with location
Because g is different in different places, weight changes too. For example, the Moon's gravitational field strength is about 1.6 N/kg, so an object weighs about 6 times less on the Moon than on Earth — but its mass stays the same.
Centre of mass
The weight of an object is considered to act at a single point called its centre of mass. Weight is measured using a calibrated spring balance (a newtonmeter).
Weight and mass are directly proportional
Since g is constant in a given location, weight is directly proportional to mass.
Exam tips
- Mass (kg) is constant; weight (N) depends on gravity.
- Learn W = mg and that g ≈ 9.8 N/kg on Earth.
- Weight acts at the centre of mass.
- On the Moon, mass is unchanged but weight is less (smaller g).